The present invention relates to a high speed optical transmission system and, more particularly, to an optical transmission system utilizing optical phase conjugation with included Raman amplification to reduce the presence of four-wave mixing and other Kerr effect nonlinearities in the transmission fiber.
In optical communication systems which utilize optical fiber as a transmission medium, chromatic dispersion and fiber nonlinearities present significant obstacles to achieving higher system data rates and longer repeater-less transmission distances. Chromatic dispersion, often simply referred to as xe2x80x9cdispersionxe2x80x9d, refers to a phenomenon in which the speed of an optical signal through an optical transmission medium (such as fiber) varies as a function of the optical signal wavelength. The problem of chromatic dispersion is particularly significant in the standard single mode fiber (SMF) making up much of the world""s existing optical transmission system infrastructure. Standard SMF typically exhibits a dispersion zero at a wavelength of about 1330 nm, with positive dispersion for wavelengths longer than the dispersion zero.
Dispersion can be expressed in terms of variations in the propagation constant of the fiber with respect to frequency. First- and second-order group velocity dispersion refer to the second and third derivatives of the fiber propagation constant xcex2 with respect to angular frequency xcfx89, or xcex22 and xcex23, respectively. Higher order dispersion terms can be approximated as zero in most applications. When used in the context of lightwave transmission systems, first- and second-order dispersion are commonly expressed in terms of derivatives with respect to wavelength. Thus, first-order group velocity dispersion is typically expressed as a change in pulse propagation time over a unit length of fiber with respect to a change in pulse wavelength. In this case, the symbol D(xcex) is often used to refer to first-order group velocity dispersion, and the units are typically picoseconds per nanometer-kilometer (ps/nm-km). Second-order group velocity dispersion is then expressed, using units of xcexps/nm2-km, as the derivative with respect to the wavelength of D(xcex).
Besides chromatic dispersion, Kerr-effect non-linearities inherent within the glass fiber can limit its transmission capabilities. In these non-linearities, the index of refraction increases with the intensity of an applied optical signal. Changes in the fiber index of refraction modulate the phase of an optical signal passing through the fiber, and thereby redistribute the signal frequency spectrum. In multi-channel systems, in which one signal causes modulation of other signals, this phenomenon manifests itself as unwanted spectral sidebands surrounding the signal wavelength. These non-linearities are usually classified as four-wave mixing (FWM), self-phase modulation (SPM) and cross-phase modulation (XPM). For long distance communication over optical fiber, dispersion and nonlinearities must be controlled, compensated, or suppressed.
Furthermore, these nonlinearities become even worse as the optical power launched into the fiber increases. As the information carried along the optical fibers is modulated at faster and faster rates, the power being used per channel rises, with a corresponding worsening of optical nonlinearities. At the same time, fibers with low dispersion are also being widely deployed and optical systems with dense wavelength division multiplexing (DWDM) are viewed as the solution for an increasing need in information capacity. These two last factors also contribute to exacerbate the generation of the above-mentioned unwanted spectral sidebands due to FWM. Moreover, XPM and SPM penalties also increase when fiber with low dispersion is used, as well as when the channel spacing is reduced. Techniques for lowering the optical power present in these sidebands and to reduce those nonlinearities are thus highly desirable for optical telecommunication systems.
One prior art technique for overcoming the presence of these nonlinearities is the use of mid-span optical phase conjugation. Because the phase conjugate of an optical pulse is, in effect, a time reversal of the pulse, an optical phase conjugator placed at the midpoint of an optical fiber span allows the first-order group velocity dispersion of the first half of the span to be compensated by the identical first-order group velocity dispersion produced as the conjugated signal propagates along the second half of the span. U.S. Pat. No. 5,798,853 issued to S. Watanabe on Aug. 25, 1998 describes one such prior art optical phase conjugation arrangement. As discussed, mid-span optical phase conjugation (OPC) can reduce the overall non-linearities in the fiber, based on the same time reversal argument, as long as the absorption in the fiber is low.
A remaining problem with this and other prior art solutions to the fiber nonlinearity problem is that optical phase conjugation is only applicable in situations where the fiber absorption is low. Since absorption is naturally a function of the length of the fiber, the prior art optical phase conjunction technique is best suited for short-span situations. Since the industry trend is toward longer and longer spans (and since nonlinearities are, in fact, more problematic for longer spans), a need remains for addressing the fiber nonlinearities in long-haul communication systems.
The need remaining in the prior art is addressed by the present invention, which relates to an optical transmission system utilizing optical phase conjugation with included Raman amplification to reduce the presence of four-wave mixing and other Kerr effect nonlinearities in the transmission fiber.
In accordance with the teachings of the present invention, the phase conjugation compensation is improved by inserting Raman gain in each fiber span (or in another embodiment, in alternate fiber spans) so as to provide for symmetric power distribution along the length of the fiber. By providing this gain in the specified spans, four-waving mixing and other nonlinearities are significantly reduced.
In a preferred embodiment of the present invention, each Raman amplification signal is applied as a counter-propagating signal with respect to the propagation direction of the information signal(s). Alternatively, counter-propagating Raman pumps can be used in only the fiber spans that follow the OPC device.
It is an aspect of the present invention that the Raman amplification technique for providing symmetrical power distribution surrounding an optical phase conjugator can be used with virtually any conjugator arrangement.